Computed Tomography (CT) is a tool used to plan modem radiation therapy. Under direction of an oncologist, a CT device generates multiple x-ray images of a patient and assimilates the images into a two-dimensional cross-sectional CT image of the patient's body. Unlike traditional x-ray images, a CT image depicts both hard objects such as bone and soft tissue including tumors. As a result, the CT image may be used for diagnosis, to delineate diseased tissue and healthy organs-at-risk, to define a treatment isocenter, and to design properties of a radiation beam usable to treat the patient (e.g., beam type, shape, dosage, duration).
CT virtual simulation gives clinicians the flexibility needed to treat the tumor, while avoiding organs-at-risk. This is done by graphic simulation of the treatment process and designing the optimum scenario for the treatment. The use of CT simulation improves the accuracy of treatment planning. More accurate planning puts a heavy demand on accurate patient positioning. In order to create a CT image, the patient is carefully positioned so as to permit x-ray radiation emitted by the CT device to intercept only an area of the patient's body that is of interest, and to avoid tissue in other areas. Immobilization devices and radiation shields are often used to achieve these ends.
Laser projectors provide one method of marking of the patient. The marks placed on patient skin are then used for the placement of patient under the dose delivery system. Laser making relies on a few points for patient alignment. The alignment of these few points ensures the correct placement of the patient as a whole; however, this technique fails to account for body deformations that often occur during transport of the patient. This problem often occurs during treatment of obese patients, and also for the treatment of the breast. For example, it is important to reposition the patient in such a way that a compliant breast is the exact shape as it was while the patient was on the CT table.
Treatment plans are designed to maximize radiation delivered to a target while minimizing radiation delivered to healthy tissue. However, a treatment plan is designed assuming that relevant portions of a patient will be in a particular position during treatment. If the relevant portions are not positioned exactly as required by the treatment plan, the goals of maximizing target radiation and minimizing healthy tissue radiation may not be achieved. More specifically, errors in positioning the patient can cause the delivery of low radiation doses to tumors and high radiation doses to sensitive healthy tissue. The potential for misdelivery increases with increased positioning errors.